136 



VEEMES 



PHYLUM III 



sometimes flattened, sometimes cylindrical. According as the internal 

 Segments correspond exactly with the external, or as each internal segment 

 corresponds to a definite number (3, 4 or 5) of the external rings, two 

 classes, Chaetopoda and Hirudinea, are distinguished. A further difference is 

 to be noticed in the locomotive organs, the Chaetopoda having bristle-bearing, 

 unjointed appendages (parapodia) on each ring of the body ; and the Hirudinea 

 having a terminal sucker. The latter group includes only the Leeches, which 

 are not known with certainty in the fossil State. Fossil representatives of the 

 third class, the Gephyrea, Annelids with the body devoid of any appearance 

 of segmentation in the adult condition, are known ; but of the fourth and last 

 class Archiannelida, the most primitive of all living Annelids, no fossil remains 

 have been found. 



Class 1. CHAETOPODA. (Earthworms, Annelids, etc.) 



It is only with the subclass of mari>ne worms (Polychaeta) that the 

 paleontologist is concerned since the earthworms and their allies (Oligochaeta) 

 are wholly unknown as fossils. The marine Chaetopoda are divisible into 

 three Orders, the Miskoa, the Tubicola or Sedentary Worms, and the Nereid 

 or Errant Annelids. 



Order 1. MISKOA Walcott. 



Polychaeta with similar segments and parapodia throughout the length of the hody ; 

 retradile prohoscis ; straight enteric canal. Body not distinctly speciaUsed into 

 sections. 



This Order is founded upon a remarkable series of Annelids discovered by 



Fig. 214. 



Wiwaxia corrvgata Walcott. Middle 

 Cambrian ; British Columbia. Crushed 

 specimen showing displaceü spines and 

 scales, X i/i (after Walcott). 



i'Kis. zia aiid 216. 



Cambrian Polychaeta from British Columbia (after 

 Walcott). Canadia spinosa and Aysheaia ■pedunculata 

 Walcott, both x ^/a- 



Walcott in the Middle Cambrian of British Columbia and described by him in 

 1911. The Order is represented by four families, namely Miskoidae and 

 Aysheaidae, with the genera Mislwia and Aysheaia respectively ; Canadidae 

 including Canadia and Selkirkia ; and the Wiwaxidae with the three genera 



